The invention relates to a comparator circuit for triggering a switching event when a rising or falling electrical signal passes through a switching threshold. The comparator circuit comprises a signal input and at least one signal output and has a first switching state and a second switching state, wherein the first switching state represents a level of the input signal lying above the switching threshold and the second switching state represents a level of the input signal lying below the switching threshold.
Comparator circuits of this kind can be used in many applications. Fundamentally, they are suited to establishing whether the level of an input signal lies above or below a pre-given threshold value.
A problem with known comparator circuits is that they suffer from hysteresis as a result of measures taken to eliminate harmful influences of noise signals.
When the input signal is hovering around the threshold value and additionally has for example noise or interference superposed on it, the signal is continually passing through the threshold value. In order to prevent the subsequently resulting multiple switching of the comparator arrangement, the comparator circuit is given an hysteresis, wherein in the amplitude of the hysteresis is tailored to the amplitude of the noise signal. A comparator circuit of this kind switches on whenever the upper hysteresis value lying above the threshold value is exceeded and switches off whenever the lower hysteresis value lying below the threshold value is undershot. Consequently, a fluctuation of the input signal between the upper and lower hysteresis values caused by an interference signal does not cause any undesirable switching events.
As well as this desired effect, there results also however the disadvantage that the on-switching level of the comparator circuit is not equal to its off-switching level.
That is to say, known comparator circuits afflicted with hysteresis display a different switching behaviour depending upon the side from which the input signal approaches the switching threshold. This brings considerable associated problems for electronic switching arrangements when it is required or desirable that switching occurs exactly at a pre-given switching threshold independently of the sign of the change in the input signal.
In particular, the use of known comparator circuits afflicted with hysteresis is disadvantageous when they are used in a light detector serving for signalling a traversing object. In this case, depending on the direction from which the object comes, input signal changes of different sign or alternatively falling and rising input signals are supplied to the comparator circuit. Therefore, known comparator circuits switch at different switching thresholds depending on the direction from which the object comes.